Trace removal of benzene vapour using double-walled metal–dipyrazolate frameworks

In principle, porous physisorbents are attractive candidates for the removal of volatile organic compounds such as benzene by virtue of their low energy for the capture and release of this pollutant. Unfortunately, many physisorbents exhibit weak sorbate–sorbent interactions, resulting in poor selectivity and low uptake when volatile organic compounds are present at trace concentrations. Herein, we report that a family of double-walled metal–dipyrazolate frameworks, BUT-53 to BUT-58, exhibit benzene uptakes at 298 K of 2.47–3.28 mmol g−1 at <10 Pa. Breakthrough experiments revealed that BUT-55, a supramolecular isomer of the metal–organic framework Co(BDP) (H2BDP = 1,4-di(1H-pyrazol-4-yl)benzene), captures trace levels of benzene, producing an air stream with benzene content below acceptable limits. Furthermore, BUT-55 can be regenerated with mild heating. Insight into the performance of BUT-55 comes from the crystal structure of the benzene-loaded phase (C6H6@BUT-55) and density functional theory calculations, which reveal that C–H···X interactions drive the tight binding of benzene. Our results demonstrate that BUT-55 is a recyclable physisorbent that exhibits high affinity and adsorption capacity towards benzene, making it a candidate for environmental remediation of benzene-contaminated gas mixtures.


Section 2. Comparison of synthetic conditions
Based on the H2BDP ligand, four supramolecular isomers, including the single-walled Co(BDP) and Zn(BDP), and double-walled isomers BUT-55 and BUT-58, have synthesized, respectively. The synthetic conditions are summarized in Table S4 for comparison. It was found that less water favors Co(BDP) and Zn(BDP), whereas more water and higher temperature favors BUT-55 and BUT-58.

Section 3. X-ray Crystallographic Analysis
The datasets were corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. [S2] All the structures were solved using direct methods and refined by full-matrix least-squares on F 2 with anisotropic displacement using the SHELXTL software package. [S3] Non-hydrogen atoms on the frameworks were refined with anisotropic displacement parameters during the final cycles. The hydrogen atoms on the ligands were positioned geometrically and refined with a riding model. The electron density of the disordered guest molecules was flattened according to the SQUEEZE routine in PLATON. [S4] Crystal parameters and structure refinement of BUT-53 to BUT-58 are summarized in Table S1 (for details, see CCDC 2085589-2085594, 2118113).
Single-crystal structure analysis on H2O@BUT-55: The single-crystal structure of H2O@BUT-55 suggests that there should be 1.5 water molecules absorbed per formula at site I, corresponding to 38% of water uptake (5.5 mmol) as reflected by the water sorption isotherm (maximum uptake of 14.4 mmol). Site II must therefore account for the remaining 8.9 mmol of uptake even though no absorbed water molecules were observed in the SCXRD structure H2O@BUT-55. We recollected the SCXRD data at 100% RH, but obtained similar results (high crystallographic disorder of water molecules at site II). Simulated distribution of adsorbed water molecules in the unit cell of BUT-55 also supports the presence of water molecules at site II (Fig. S44).
In summary, water sorption uptake and simulations support the presence of water molecules at both sites I and II.

Section 4. Gas Adsorption
Benzene adsorption isotherms shown in Fig. S19 were collected on a Micrometrics ASAP 2020 instrument using default equilibrium parameters (pressure change over 100 s of less than 0.01% of the average pressure during the interval).
Data collection for each sample require more than three days.

Section 5. In situ powder X-ray diffraction
In situ variable pressure PXRD data were recorded on a Rigaku Smart Lab instrument operated at 40 kV and 40 mA with an Anton Paar TTK 600 accessory. The PXRD patterns were collected under benzene loading from 0 to 8.53 kPa at 32 °C.
The sample was activated at 120 °C under vacuum condition before testing.
In situ variable temperature PXRD data were recorded on a PANalytical X'Pert

Section 6. Breakthrough Experiments
After the content of outlet gas reached equilibrium, the adsorption bed was regenerated by N2 flow (20 mL/min) for 2 hours at 120 o C.
To evaluate the behavior of materials under real conditions, BUT-55 and BUT-58 were selected to check their benzene adsorption performance after being previously exposed to moisture. The adsorption experiments were performed by exposing BUT-55 and BUT-58 samples to the atmosphere one week before testing (RH ranging from 35 to 64%, at 9 am from 3 rd to 10 th Oct in Beijing).
The concentration of benzene in effluent has been analysed with a GC-MS instrument (Clarus 600 GC-MS (Perkin Elmer, U.S) instrument coupled with a Turbomtrix350 TD (Perkin Elmer, U.S)). Samples are collected into a Tenax TA sorbent tube for detection, and the mass of analytes could be calculated by referring to the standard curve with a peak area value. Specifically, LOD of this method can be determined by testing seven samples with a similar analyte mass (ng), identifying the specific benzene mass of each sample by referring to the standard curve, and calculating the standard deviation of these mass values. LOD is finally obtained by multiplying the standard deviation with a coefficient 3.14. For benzene in the present research, the standard deviation was calculated to be 1.599 ng by taking seven samples with benzene mass of around 100 ng ( Fig. S32 and Table S2), and the LOD was finally determined to be 5.02 ng. As indicated by this LOD value, if we prepare a sample containing benzene more than 5 ng, the detection would be reliable.
Because the concentration of benzene in effluent streams is very low, we tried to continuously collect effluent for a long period to prepare large volume samples.

Section 7. First-Principles Based Computational Details
DFT calculations were carried out to explore the active sites and reaction mechanisms using the CP2K code. [S5] All calculations employed a mixed Gaussian and plane-wave basis sets. For the core electrons, norm-conserving Goedecker-Teter-Hutter pseudopotentials [S6-S9] were adopted. The valence electron wave function was expanded in a double-ζ basis set with polarization functions [S10] along with an auxiliary plane wave basis set. Perdew, Burke, and Enzerhof (PBE) exchange-correlation functional [S11] within the generalized gradient approximation was applied. The cutoff energy was set to 400 Ry. The -point only sampling scheme was used in all calculations. The reaction state configurations were optimized with the Broyden-Fletcher-Goldfarb-Shanno (BGFS) algorithm with SCF convergence criteria of 1.0×10 -8 au. To compensate the long-range van der Waals dispersion interaction between the adsorbates and MOF skeleton, the DFT-D3 scheme [S12] with an empirical damped potential term was added into the energies obtained from the exchange-correlation functional in all calculations.
The adsorption energy between benzene and the BUT-55 substrate can be calculated using the following equation: In Eq.
(1), @ −55 and −55 represent the total energies of benzene with and without BUT-55, respectively. is the energy of the benzene.
According to this equation, a negative adsorption energy corresponds a stable adsorption structure.
For BUT-53, -54, and -58, the adsorption energy between benzene and the BUT substrate can also be calculated using the same equation with (1), respectively.
The DFT calculations are used for explanation of the benzene binding. Among these MOFs, BUT-53 and BUT-55 have similar binding energies, but their adsorption properties are different. the kinetics of benzene adsorption on BUT-53 and BUT-55 using thermogravimetric analysis (TGA) (Fig. S43b). These data reveal that benzene S14 is absorbed slightly slower by BUT-53 than by BUT-55. The adsorption performance of BUT-53 vs. BUT-55 can be described as follows: (1) At ultra-low pressure (the first few data points of the Henry's region), the isotherm of BUT-53 exhibited higher uptake and a stepper gradient than that of BUT-55 (Part 1 in Fig. S43a) In addition, the SCXRD data reveal that a minor ligand swing was observed before benzene adsorption in BUT-53, which was immobilized by benzene adsorption (Fig. S43c). This guest induced structural immobilization has not been observed in the other BUT sorbents. This may also contribute to their different adsorption performances.